Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...

"Sushi Chef" Rank:In any level, use the knife to kill as many people as you can.

"Pianoman" Rank:Kill the majority of your victims using Fiberwire.

Free Cash: This exploits a bug in restarting a replay mission, so it may sound confusing. The idea is to capitalize on the reward you get for re-doing a mission better than you did before, earning more cash. The positive balance is always transfered into your account. Complete a level very poorly. Then, replay the level and do as well as you can. Before you complete the mission, create a save point near the end. When looking at the mission review screens, after your reward is added, press B to return to the first screen and Y to replay the mission. The game says you'll lose your progress, but you should start at the save point you created before you escaped the mission. End the mission again, and collect your reward again. Repeat.

Free Intel: To save some cash on intel, start a mission and purchase all the available intel. Once in the mission, make a note of the information and choose to restart the mission. You'll have the money back you spent on the intel, and if you paid attention, you'll still have all the info.

Alternate Title Screen: Beat the game to see an alternate version of the title screen.

Controllable Ending: After the last level, watch the credits. Afterwards, when 47 is being lowered into the ground, you'll hear his heartbeat and see the life gauge moving in time with it. Rotate the Left Analog Stick repeatedly, and notice the health bar increase as the heartbeat speeds up. Keep going, and once you fill up the meter, 47 will jump up and you'll be able to control him. Now, in true Hitman fashion, leave no witnesses!

Get Down: In the mission 'A Murder of Crows', there are three different bars (Rock, Salsa, Blues). You can score costumes that fit in at one of the bars. So, get to any of the bars and whip out the corresponding costume, leave 47 on the dance floor for a minute, and watch him shake, shake it.

Rat Club: In the mission "Curtains Down", head to the basement. There is a room with 3 rats. Kill them and one will drop a keycard. Proceed to the top floor and use that card on the only locked door you find. Inside will be a bunch of rats boxing and playing cards!

Achievements:

First Mission Complete (25 points): "Death of a Show man" completed.

Rookie Mode Complete (25 points): Complete the game under the Rookie difficulty.

Normal Mode Complete (50 points): Complete the game under the Normal difficulty.

Expert Mode Complete (75 points): Complete the game under the Expert difficulty.

Professional Mode Complete (150 points): Complete the game under the Professional difficulty.

47 Kills In A Mission (20 points): A mission has been completed with exactly 47 kills.

Notorious (20 points): 47 is the World's Most Wanted.

Redemption (50 points): At the end of the game rapidly flick the Left Analog-stick in any direction when you are lying on your death bed. When you wake up, kill everybody so that there are no witnesses.